In a fuel injection system, fuel is accumulated in a common rail as a pressure-accumulation vessel, and a fuel injection valve injects the fuel in accordance with an injection instruction signal. In such a fuel injection system, fuel may be injected in a different mode from an injection instruction due to fuel leak or the like. For example, JP-A-5-52146 discloses a device for detecting such a defective injection state. In the fuel injection system according to JP-A-5-52146, the common rail is provided with a rail pressure sensor for detecting pressure of pressure-accumulated fuel. In the present system, an operation of a fuel pump for feeding fuel to the common rail is feedback-controlled such that the detected pressure of the rail pressure sensor coincides with a target value. The target value is determined on the basis of rotation speed of the engine and engine load. The defective injection detection device according to JP-A-5-52146 determines whether the target value is less than a reference value due to fuel leak or the like. The defective injection detection device detects a defective injection state when determining the target value to be less than the reference value, i.e., the injection quantity to be less than demanded quantity.
However, the defective injection detection device according to JP-A-5-52146 detects the defective injection when determining a failure to be caused in the target value, which is used in the feedback control. Accordingly, the present defective injection detection device indirectly detects the actual injection state. Therefore, a time lag between a time point, at which the fuel injection quantity actually begins to decrease due to fuel leak or the like, and a time point at which a failure occurs in the target value, is large. Therefore, quick detection of the defective injection is difficult, and accuracy of the detection of the defective injection is also low.